The present invention relates to the manufacture of steel bar stock and, more particularly, to a method and apparatus for pack cooling flat stock on the cooling bed of a rolling mill.
In recent years, smaller, more efficient steel mills, sometimes called mini-mills, have become an increasingly important segment of the domestic steel industry due to their ability to economically compete with the manufacturers of imported steel. Such mini-mills usualy manufacture specialty products including flat, round, angle and assorted merchant stock of various cross-sections.
Regardless of the type of bar stock manufactured, it is desirable to slow cool the product in order to relieve residual inner stresses formed by non-uniform cooling and to render the stock softer and, thus, more suitable for cold working. After hot-rolling, the stock is air-cooled. This type of cooling results in formation of pearlite, which is normally soft. However, to obtain these results, the cooling rate must be sufficiently retarded in order to prevent the formation of bainite or martensite.
For merchant stock and angle or round stock, cooling beds having a plurality of both movable and stationary notch bars or rakes have been utilized. Such cooling beds may be 25 to 30 feet in width and up to 280 feet long. In operation, the bar stock is moved along the length of the cooling bed by reciprocating the movable notch bars with respect to the stationary notch bars. Consequently, the stock is lifted off the notches of the stationary bar by the notches of the moving bar and is then placed in the succeeding notches of the stationary bars. Cooling of the stock as it crosses the bed is aided by an updraft.
While a standard cooling bed of the type described above has proved satisfactory for use in cooling merchant stock of various cross-sections, as well as angle and round stock, a standard cooling bed does not provide a sufficiently slow cooling rate to prevent the formation of bainite or martensite when thin flat stock is being cooled by transporting such thin stock across the cooling bed one at a time. This has prompted the use of pack or stack cooling beds in which a number of flat bars are stacked before being air cooled on the bed. Typically, the flat stock is stacked with the most recently finished flat bar being placed on the top of the stack. By stacking the flat bars, their cooling rates are sufficiently reduced, particularly in the areas around the peripheries of each of the flat bars in the stack, to prevent the formation of bainite or martensite. Once a stack is formed in the initial row of notches in the pack cooling bed, the moving notch bars are activated to transport the stack across the bed.
However, just as a standard cooling bed is unsuitable for the slow cooling of flat bars, a pack cooling bed is unsuitable for the slow cooling of most merchant stock and angle and round stock because the cross-sections of such stocks preclude stacking. Thus, mills have had to choose between manufacturing only flat bars or merchant stock, round stock and angle stock or making substantial additional capital outlays for both a standard cooling bed and pack cooling bed in order to be able to manufacture all types of bar stock.
Accordingly, it is the primary object of the present invention to provide an improved cooling bed that may be utilized in the process of slow cooling stock of various cross-sections, including flat bars, angle stock, round stock and merchant stock.
More particularly, it is an object of the present invention to provide a cooling bed that permits bar stock, regardless of its cross-sectional configuration, to cool at a sufficiently slow rate to effect desired formation of pearlite in the stock.
Other objects and advantages may be appreciated upon reference to the following detailed description and the accompanying drawings.